The majority of the water in Earth's oceans is incredibly ancient, estimated to be approximately 4 billion years old.
The Age of Ocean Water
Scientists have determined that most of the water on our planet, including the oceans, is extremely old. While the precise origin and timing of the ocean's formation are still being researched, evidence suggests that the water molecules we find in our oceans today have existed for billions of years. One study suggests the presence of water as far back as 4.28 billion years ago. https://en.wikipedia.org/wiki/Origin_of_water_on_Earth
The process of the ocean's formation is complex, starting with the Earth cooling enough for water vapor to condense and form liquid water. https://oceanservice.noaa.gov/facts/why_oceans.html This early ocean was likely quite different from the one we know today, both in terms of chemistry and overall volume, but the water itself is incredibly old. https://www.nature.com/scitable/blog/saltwater-science/how_old_is_the_ocean/
Therefore, while the ocean as a body of water has evolved over time, the water itself is extraordinarily ancient.
